The present invention relates generally to superconducting magnetic energy storage (SMES) units, and more particularly to a superconducting magnetic energy storage unit wound in a toroidal fashion such that the magnetic field is contained within the bore of the magnet, thus producing a very low external field.
Superconducting magnetic energy storage (SMES) units have a number of advantages as storage devices. Electrical current is the input, output, and stored medium, which allows for complete solid-state energy conversion. The magnets themselves have no moving parts. Round-trip efficiency is higher than those parts for batteries, compressed air, or pumped hydro. Output power can be very high, allowing complete discharge of the unit within a few seconds. Finally, the unit can be designed for a very large number of cycles, limited essentially by fatigue in the structural components. Basically, two types of magnets have been studied: the solenoid, a circular loop of conductor; and the toroid, in which the circular or D-shaped coils are arranged in a circle to produce a doughnut-shaped field. A solenoid produces a magnetic field, B, which is external of the magnet, whereas a toroid produces a magnetic field that is contained within the magnet's bore.
Present U.S. SMES designs foresee stationary machines having a vertical-axis solenoidal magnet located underground. Large solenoidal magnets produce a significant external field and thus preclude mobile and space-borne applications. In mobile uses, the external field would induce eddy currents in nearby conducting structures such as bridges, reinforcing rods and light poles, thereby dissipating some of the stored energy. In addition, members of the public wearing pacemakers or other field-sensitive devices may be nearby and possibly effected. In space applications, the external field of the solenoid would funnel charged cosmic particles to the spacecraft, exposing the crew and instrumentations to higher radiation exposures. Additionally, large solenoidal coils must be wound on site and cannot generally be manufactured as transportable modules in a factory.
The discovery of high temperature superconductors and improvements in conventional superconductors has generated renewed interest in the use of intense magnetic fields for energy storage using either solenoids or toroids. The relative merits of these types of magnets have been compared, and it has generally been concluded that a solenoid is preferable in terms of energy stored per unit mass of superconductor and structure. Most of the SMES design studies in the United States has focussed on solenoids.
However, toroidal magnets, which require approximately twice as much superconductor and structure per unit of stored energy as a solenoid, have several advantages. Because the field is closed within the bore of the magnet, the external field is very small. Generally, the external field is a product only of the toroid being composed of several individual coils, rather than a continuous current sheet. Leads and winding irregularities also produce small external fields. In mobile applications where the external field would produce eddy currents in roadside conductors, or in urban stationary applications where the external field would have to be limited, a toroidal magnet may be more appropriate. Additionally, a toroidal magnet is much more amenable to factory winding and fabrication. A toroidal SMES can be composed of several individual coils which would be fabricated and tested in a factory, transported by truck or rail to the site, and placed to form a circular arrangement. Thus, by approaching an ideal toroidal current sheet, co-locating current leads and minimization of winding irregularities, the external field can be reduced to very low levels.
Magnetic energy storage units have many applications, an example of which is a buffer for the kinetic energy of an automobile, bus, or railcar. In this example, about 30 kW of power is required to comfortably accelerate a small automobile into traffic, but only about 8.5 kW is required to maintain the automobile at 55 mph on level pavement. Therefore, an automobile engine spends most of its time operating relatively inefficiently. Additionally, the present internal combustion engine does not allow for regenerative braking. Although battery-powered automobiles have certain advantages to offset the problems of the internal combustion powered automobile, they are often hampered by poor acceleration due to limitations on instantaneous current. Battery lifetime is limited by the total number of cycles and the battery has a limited ability to absorb energy in regenerative braking. One possible way to circumvent these difficulties is to use a short-term, high specific power energy storage device.
Accordingly, it is an object of the present invention to provide a toroidal superconducting magnetic energy storage unit such that the magnetic field produced is contained within the bore of the magnets and in which the external magnetic field is very low.
It is another object of the present invention to provide a toroidal superconducting magnetic energy storage unit which does not rely on external structures for support, except for normal gravity and inertial forces.
Yet another object of the present invention is to produce self-consistent designs for toroidal superconducting magnetic energy storage units.
A further object of the present invention is to provide an SMES which has wide applicability, from providing a buffer for the kinetic energy of a car, bus, or railcar, to providing a power source for an electromagnetic launcher.